1. Field of the Invention
The present invention relates to an automatic choke apparatus for an engine, the automatic choke apparatus utilizing heat of an exhaust system so as to operate a choke valve in an intake system.
2. Description of Related Art
A choke valve is provided to a carburetor, which constitutes an intake system of an engine, in order to enhance starting performance of the engine at a low temperature. An intake air amount is reduced by closing the choke valve, whereby an air-fuel mixture can be thickened. Accordingly, the engine can satisfactorily be started. As described above, when the engine is started, the choke valve is closed to thicken the air-fuel mixture. Therefore, it is necessary to open the choke valve so as to appropriately adjust the air-fuel mixture after the warm-up of the engine.
In order to automate such a operation choke valve, an automatic choke apparatus that operates a choke valve by a bimetal has been proposed (see, for example, Japanese Patent Application Laid-Open No. H7-145757). This automatic choke apparatus has a bimetal in the vicinity of a muffler, and operates the choke valve using the bimetal in a closing direction when an engine is started (when the muffler is at a low temperature), while it operates the choke valve in an opening direction after the warm-up of the engine (when the muffler is at a high temperature).
As described above, the choke valve has to be rapidly opened after the completion of the warm-up in order to appropriately adjust the air-fuel mixture supplied to the engine. However, it is difficult to open the choke valve according to the completion of the warm-up, depending upon a mounting condition of a bimetal. Specifically, exhaust gas flowing in the muffler is guided from an upstream expansion chamber toward a downstream expansion chamber, and immediately after the start of the engine, there is a great difference in a temperature distribution in the muffler. The mounting condition of the bimetal disposed in the vicinity of the muffler is limited by a layout of an intake/exhaust system or a shape of an engine. When there is a great variation in the temperature distribution in the muffler, and the mounting position of the bimetal is limited, it is impossible to mount the bimetal at a high-temperature portion of the muffler. Therefore, it is difficult to rapidly heat the bimetal after the start of the engine. The delay in the release of the choke valve due to the delay in heating the bimetal might cause an incomplete combustion of the engine due to excessive fuel. Therefore, it has been desired that the bimetal is rapidly heated, regardless of the mounting position of the bimetal.